The present invention relates to a method for depositing a nodule-free coating layer on a substrate surface by means of an electroless plating process. The invention has particular utility in depositing amorphous nickel-phosphorus (NiP) layers on suitably-shaped substrates, e.g., disk-shaped substrates, utilized in the manufacture of longitudinal magnetic recording media.
Magnetic recording media are widely used in various applications, particularly in the computer industry. A conventional longitudinal magnetic recording disk medium 1 used in computer-related applications is schematically depicted in cross-sectional view in FIG. 1 and comprises a non-magnetic substrate 10 selected from metals, metal alloys, polymers, polymer-based materials, glass, ceramics, metal-ceramic composite materials, and glass-ceramic composite materials, typically an aluminum (Al)-based alloy, such as an aluminum-magnesium (Alxe2x80x94Mg) alloy, having sequentially deposited on at least one surface 10A thereof: a xe2x80x9cseedxe2x80x9d or plating layer 11, typically of an amorphous nickel-phosphorus material, such as NiP and Ni3P; a polycrystalline underlayer 12, typically of chromium (Cr) or a Cr-based alloy; a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy; a protective overcoat layer 14, typically comprised of diamond-like carbon (DLC); and a lubricant topcoat layer 15, typically comprising a perfluoropolyether compound.
According to conventional automated manufacturing methodology for fabricating such type magnetic recording media, each of the polycrystalline underlayer 12, magnetic recording layer 13, and protective overcoat layer 14 is deposited on, e.g., an amorphous NiP- or Ni3P-plated substrate, by a suitable physical vapor deposition (PVD) or chemical vapor deposition (CVD) technique, typically cathode sputtering. When utilized with relatively soft substrates, such as Alxe2x80x94Mg alloy substrates 10, the NiP plating layer 11 is typically deposited by an electroless plating process to form a layer having a thickness of about 15 xcexcm, in order to increase the hardness of the substrate surface, thereby providing a suitable surface for subsequent polishing and/or texturing. The presence of amorphous NiP or Ni3P xe2x80x9cseedxe2x80x9d or plating layer 11 is also necessary for ensuring proper polycrystallinity of the Cr-based underlayer 12, which, in turn, is required for facilitating proper epitaxial growth thereover of a suitably polycrystalline magnetic recording layer 13. For example, an amorphous NiP or Ni3P xe2x80x9cseedxe2x80x9d layer 11 induces a Cr-based underlayer 12 deposited thereon to exhibit a (200) crystallographic orientation, which, in turn, causes the magnetic recording layer 13 deposited and epitaxially grown thereon to exhibit an advantageous bi-crystal cluster microstructure, as disclosed in U.S. Pat. No. 5,733,370, the entire disclosure of which is incorporated herein by reference.
In some instances, the xe2x80x9cseedxe2x80x9d layer 11 is provided with a textured or roughened surface to facilitate preferential alignment of the Cr-based underlayer 12 to exhibit the (200) crystallographic orientation or to reduce xe2x80x9cstictionxe2x80x9d between the transducer head and the recording medium when in use. In other instances, a requirement for substrates with high track-per-inch (xe2x80x9cTPIxe2x80x9d) and low track mis-registration (xe2x80x9cTMRxe2x80x9d) necessitates formation of NiP or Ni3P xe2x80x9cseedxe2x80x9d or plating layers 11 with defect-free surfaces after plating and/or polishing, with an attendant requirement for a high degree of planarity.
Suitable baths and procedures for electroless plating of non-magnetic nickel-phosphorus (NiP) amorphous xe2x80x9cseedxe2x80x9d or plating layers, wherein the formula xe2x80x9cNiPxe2x80x9d is taken to include all ratios of nickel-to-phosphorus, are disclosed in U.S. Pat. Nos. 3,531,322 and 4,659,605, the entire disclosures of which are incorporated herein by reference. By way of illustration only, a suitable electroless plating bath for deposition of amorphous NiP xe2x80x9cseedxe2x80x9d or plating layers consistent with the requirements of the present invention, includes a source of nickel ions (e.g., NiCl2 or NiSO4), a source of hypophosphite ions (e.g., NaH2PO2), a buffering agent, e.g., a carboxylic acid, boric acid or borate, and certain ester complexes, e.g., an ester complex of glucoheptonic acid, and stabilizing agents, etc. Another suitable NiP electroless plating bath includes a source of nickel ions, an unsaturated carboxylic acid, and a source of hypophosphite ions. In addition to these, electroless NiP plating baths usable within the context of the invention include, inter alia, Enthone 6450 (Enthone-OMI, New Haven, Conn.), Fidelity 4355 (OMG Fidelity Products Corp., Newark, N.J.), and U1C SHDX (Uyemura Int""l Corp., Ontario, Calif.).
NiP electroless plating baths, such as described above, can provide non- magnetic, amorphous NiP deposits, with a phosphorus (P) content within the range of from about 8 to about 12% and a corresponding nickel (Ni) content of from about 92 to 88%. Further, these baths are typically operated at an acidic pH, i.e., below about 5, and at an elevated temperature, i.e., above about 140xc2x0 F., typically about 180-200xc2x0 F., to provide a practically useful plating rate of about 3 to about xcexc inches/min. while still providing a non-magnetic deposit which does not become magnetic with age.
An essential requirement of the above-described NiP electroless plating process is that the thus-plated NiP layer be characterized by an unusually smooth surface which is free of imperfections such as nodules and pits. Prevention of formation of such imperfections is particularly important in the manufacture of rigid magnetic media, such as, for example, hard disks, since irregularities of any kind in excess of one-millionth of an inch can cause head crash or defective recording.
In practice, however, the requisite freedom from formation of surface irregularities during electroless plating of non-magnetic, amorphous NiP xe2x80x9cseedxe2x80x9d layers, such as the above-mentioned nodules and pits, frequently is not achieved in continuous manufacturing processing for the fabrication of magnetic recording media, e.g., hard disks, leading to increased substrate rejection rates. For example, abnormal nodule growth is frequently observed when less costly, more readily-available materials, e.g., polymeric or polymer-based materials, are utilized as components of the NiP electroless plating line in order to reduce or minimize equipment expense. Such abnormal nodule growth can result in the presence of residual xe2x80x9cbumpsxe2x80x9d after post-deposition polishing of the NiP layer or add to the manufacturing cost by necessitating a two-step polishing process to ensure complete nodule removal. Moreover, in instances where a leveling agent is added to the NiP electroless plating bath to produce smooth layers, the effect of any abnormal nodule growth will be exacerbated.
Accordingly, there exists a need for an improved electroless plating process suitable for forming defect-free xe2x80x9cseedxe2x80x9d or plating layers for use in the manufacture of high density magnetic recording media, which xe2x80x9cseedxe2x80x9d or plating layers are substantially free of abnormal nodule growth and require little or no post-deposition polishing prior to subsequent layer deposition thereon. In addition, there exists a need for an improved electroless processing methodology for manufacturing substrates for high-density magnetic recording media which is simple, cost-effective, and fully compatible with the productivity and throughput requirements of automated manufacturing technology.
The present invention fully addresses and solves the above-described problems attendant upon the formation of substrates utilized in the manufacture of high-density magnetic recording media, while maintaining full compatibility with all chemical and mechanical aspects of conventional recording media manufacturing technology.
An advantage of the present invention is an improved method of electroless plating of substrates.
Another advantage of the present invention is an improved method of nodule-free electroless plating of substrates utilized in the manufacture of high-density magnetic recording media.
Yet another advantage of the present invention is an improved method of nodule- and defect-free electroless plating of NiP xe2x80x9cseedxe2x80x9d or plating layers on disk-shaped substrates utilized in the manufacture of high-density magnetic recording media.
Still another advantage of the present invention is an improved method of manufacturing a magnetic recording medium.
A still further advantage of the present invention is an improved method of manufacturing a magnetic recording medium comprising a nodule-free NiP xe2x80x9cseedxe2x80x9d or plating layer.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are obtained in part by a method of depositing a nodule-free coating layer on a substrate surface by an electroless plating process, wherein an electroless plating bath utilized for the plating is contained at an elevated temperature within a plating apparatus including at least one polymeric material, comprising performing said electroless plating process in a plating apparatus wherein the at least one polymeric material is substantially resistant to degradation by contact with the elevated temperature electroless plating bath.
According to an embodiment of the present invention, the temperature of the electroless plating bath is at least about 140xc2x0 F., and the at least one polymeric material is substantially resistant to degradation which comprises release of soluble, low molecular weight, carbon-containing species into the elevated temperature electroless plating bath, which species promote nodule growth.
According to further embodiments of the present invention, the at least one polymeric material comprises at least one fluorine-containing polymer, e.g., at least one fluorine-containing hydrocarbon polymer such as polyvinylidene difluoride (PVDF) and poly(vinylidene fluoride-hexafluoropropylene).
According to still further embodiments of the present invention, the at least one polymeric material comprises at least one fluorocarbon polymer, e.g., polytetrafluoroethylene or a derivative or composite thereof
According to yet further embodiments of the present invention, the electrolessly-plated coating layer comprises amorphous nickel-phosphorus (NiP); the substrate is a disk-shaped substrate for use in fabricating a magnetic recording medium and comprises a material selected from the group consisting of: metals, metal alloys (e.g., Alxe2x80x94Mg), polymers, glass, ceramics, metal-ceramic composite materials, and glass-ceramic composite materials.
According to another aspect of the present invention, a method of fabricating a magnetic recording medium comprises the sequential steps of:
(a) providing a disk-shaped substrate having a surface for deposition thereon; and
(b) electrolessly depositing, from an electroless plating bath maintained at an elevated temperature at least about 140xc2x0 F., a nodule-free, amorphous nickel-phosphorus (NiP) xe2x80x9cseedxe2x80x9d or plating layer on the substrate deposition surface, utilizing an electroless plating apparatus comprised of at least one polymeric material which does not release soluble, low molecular weight carbon-containing species into the elevated temperature electroless plating bath upon contact therewith.
According to an embodiment of the present invention, the method further comprises the sequential steps of:
(c) forming a polycrystalline underlayer over the NiP xe2x80x9cseedxe2x80x9d or plating layer;
(d) forming a magnetic recording layer over the underlayer;
(e) forming a protective overcoat layer over the magnetic recording layer; and
(f) forming a lubricant topcoat layer over the protective overcoat layer.
According to further embodiments of the present invention, step (b) comprises utilizing an electroless plating apparatus comprising at least one fluorine-containing polymer, e.g., at least one fluorine-containing hydrocarbon polymer such as polyvinylidene difluoride (PVDF) and poly(vinylidene-hexafluoropropylene), or at least one fluorocarbon polymer such as polytetrafluoroethylene or a derivative or composite thereof
According to yet another aspect of the present invention, a method of electrolessly depositing a nodule-free layer of a plating material on a substrate surface comprises:
(a) providing a substrate having a surface; and
(b) utilizing a means for nodule-free electroless plating of the layer of plating material on the substrate surface.
According to an embodiment of the present invention, the layer of plating material is amorphous NiP.
Additional advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.